Films of siliceous ceramics, such as silica, silicon nitride and silicon oxynitride, are excellent in heat resistance, in abrasion resistance and in corrosion resistance, and hence they are used as, for example, insulating films in semiconductor devices and liquid crystal displays and also as protective films provided on pixel electrodes or color filters therein. Among those films, a silicon nitride film is characterized by being stable at a high temperature particularly in an inert or reductive atmosphere and also by being a transparent film with a high refractive index, as compared with a silica film and the like. Accordingly, in view of the compactness and the high refractive index, the silicon nitride film is employed advantageously as a protective film or a gas barrier film in a recent optical device.
In the technical field described above, a silicon nitride film and a silicon oxynitride film (which are hereinafter often referred to as a “SiN film” and a “SiON film”, respectively) are generally formed on substrates according to chemical vapor deposition method (hereinafter, referred to as “CVD method”) or other vapor deposition method such as sputtering method.
However, a siliceous ceramic film can be also formed according to casting method in which a film-formable coating solution comprising a silicon-containing compound, such as silicon hydroxide or polysilazane, is cast on a substrate and then heated so as to oxidize and convert the silicon-containing compound into silica, silicon nitride or silicon oxynitride. For example, a process is known in which perhydropolysilazane or a denatured substance thereof is cast on a substrate and then fired at 600° C. or more in vacuum to obtain a SiN film (Patent document 1). Further, another process is also known in which a composition containing perhydropolysilazane is cast on a substrate and then converted into amorphous silicon nitride by heating at 650° C. for about 30 minutes in an inert atmosphere (Non-patent document 1).
The casting method is widely adopted because it can be performed in relatively simple facilities. However, since the heating treatment is carried out at a relatively high temperature, the cost of thermal energy is considerable and the productivity of the method is relatively poor.
The vapor deposition method is also generally adopted. However, a film formed according to the CVD method often has a surface of insufficient smoothness. In addition, if the substrate has a surface provided with grooves thereon, it is so difficult to fill the grooves in evenly that voids may be formed in the grooves.
In order to improve those problems of the vapor deposition method, it is studied that CVD procedures are carried out at a temperature of about 350° C. to form an amorphous silicon nitride film (Non-patent document 2). However, the CVD procedures, which are generally complicated, are made further complicated in this process. In addition, this process costs a lot and the productivity thereof is relatively low, and hence there are rooms for improvement.
Further, a SiN film formed according to the CVD method often gives off ammonia gas. Because of that, if the SiN film formed by the CVD method is adopted as a bottom antireflective coating on which a resist pattern is formed, the resultant resist pattern may be in the form of ridges with lower slopes. This form is referred to as “resist footing”, which is unfavorable for the resist pattern. It is hence often necessary to form a SiO film as a capping layer on the SiN film formed by the CVD method. However, if the capping layer is provided, the resist pattern may be in the form of ridges having thin bottoms. This form is referred to as “bottom pinch”, which is also unfavorable for the resist pattern. Accordingly, if the SiN film formed according to the CVD method is used as a bottom antireflective coating, the resultant resist pattern is liable to suffer from the resist footing or bottom pinch. It has been desired to improve this problem.
As for the SiN film formation method, there is an attempt to lower the temperature of the heating treatment in the casting method (Patent document 2). In this attempt, a solution of perhydro-type polysilazane is cast on a substrate and then subjected to the heating treatment at 200 to 300° C. while irradiated with UV light so as to form a SiN film. However, judging from the FT-IR spectra shown in Examples of the above document, there is probability that the formed films are not SiN films but silicon oxide films. In addition, this process is more complicated than normal casting processes. Further, although carried out at a relatively low temperature, the heating treatment is still necessary in the process. Accordingly, in consideration of reduction of the thermal energy cost, there are rooms for further improvement.